Effect of Additional Surfaces on Ordinary Portland Cement Early-Age Hydration

Early-age hydration of Ordinary Portland Cement (OPC) was studied in the presence of two additional surfaces. Additional surfaces are known to accelerate the early-age hydration of OPC. Autocatalytic reaction modelling was used to determine acceleration mechanism of additional surfaces. Heat development of the hydration was measured with semi-adiabatic calorimetry and the results were modelled with an autocatalytic reaction. Autocatalytic reaction modelling was able to determine number of initially active nucleation sites in early-age hydration. OPC hydration followed autocatalytic reaction principles throughout induction period and accelerating period. Both of the added surfaces, limestone filler and calcium-silicate-hydrate (C-S-H) coated limestone filler accelerated the early-age hydration. According to autocatalytic modelling, the C-S-H coated filler increased the number of initially active nucleation sites. Pristine limestone filler accelerated the early-age hydration by providing the additional nucleation sites throughout the early-age hydration. The difference was explained with common theories of nucleation and crystal growth. Autocatalytic model and measured calorimeter curve started to significantly deviate at the inflection point, where the reaction mode changed. The reaction mode change depended on the average particle distance. Early-age hydration, modelled as autocatalytic reaction was able to improve understanding of OPC early-age hydration and quantify the number of initially active nucleation sites. Understanding and quantifying the acceleration mechanisms in early-age hydration will aid larger utilization of supplementary cementitious materials where understanding the early-age strength development is crucial.


Introduction
In recent decades, nucleation and crystal growth mechanisms has received an ever strengthening position as the explanation of hydration phenomena of Ordinary Portland Cement (OPC) [1]. It is largely recognized that cement hydration can be divided into various stages: initial dissolution, induction period, accelerating period and deceleration period [1] [2]. Earlier, application of classical crystal growth theories [3] [4] failed to explain slow hydration during the induction period without additional hindering process. This hindering process has been explained with formation of membranes with meta-stable layers and their later rupture. During the past decade, the fundamental work of Nonat and others [5]- [12] cast doubt on the existence of membranes. Recently, Thomas adapted a modified Avrami crystal growth model [3] and successfully explained the origin of the induction period as a part of a crystallization process, without a need for additional hindering mechanism. At present, a massive body of experimental and modelling studies supports theory that a slow reaction stage, the induction period, is related to a small number of active growth sites of calcium-silicate-hydrate. Also formation of membranes is observed and modelled [13] [14]. Further evidence for nucleation models were gained from addition of artificial growth sites of calcium-silicate-hydrate by Thomas et al., in the form of a pre-precipitated calcium-silicate-hydrate (C-S-H) [15].
A number of authors indicated that the rate of OPC hydration depended on the amount of reaction products, which is analogous to the autocatalytic reaction [16] [17] [18]. It has been also observed that various fine materials accelerated hydration processes [19] [20] [21] [22] [23]. According to the autocatalytic reaction theory, the reaction product acts as catalysis for further reaction by accelerating the reaction (Equation (1)). This type of reaction exhibits an S-shaped correlation between the amounts of reaction product and the time. In the pseudo-first-order reaction, the reaction rate can be expressed as a sum of a two-stage reaction (Equation (2)): where [A] represents a concentration of initial materials and [B] represents a concentration of formed reaction products. k 1 and k 2 coefficients take into account the reaction rates. The autocatalytic reaction includes initiation of the reaction (reaction rate coefficient k 1 ) followed by the autocatalytic reaction (reaction rate coefficient k 2 ). These two reactions can be described as non-catalytic and autocatalytic, respectively. In the Ordinary Portland Cement hydration, the primary nucleation step occurs following early dissolution of tri-calcium silicate  [7]. Within the first approximately 15 min, the process consists of the dissolution of calcium and silicate up to their maximum supersaturation level along with the primary precipitation of C-S-H nuclides. This can be considered as the non-catalytic stage of reaction which later initiates the second stage reaction, the crystal growth of C-S-H. As the duration of the period of the primary nucleation step is negligible compared to the reaction period examined here, the non-catalytic-reaction in Equation (1) where [A 0 ] represents an overall initial material concentration, t is the time from initial nucleation and k 2 is the reaction rate coefficient of autocatalytic reaction.
The main objective of the present study was to analyze the effects of various surfaces on early-age hydration of Portland cement using calorimetric data combined with the autocatalytic reaction model of the C-S-H phase.

Materials and Methods
The used cement was white Ordinary Portland Cement type CEM I 52,5 R, from Aalborg. The limestones fillers, type SB63, were produced by Nordkalk Oy. The  Table 1. Particle distributions were measured with a Beckman Coulter LS particle size analyzer. The specific surface areas (SSA) were calculated from the particle size distributions ( Figure 1).
Calcium-silicate-hydrate (C-S-H) coated limestone was produced by vigorously mixing Na 2 (SiO 2 ) 3,3 and Ca(OH) 2 respectively to water in the presence of limestone filler. The used Ca(OH) 2 was purchased from Fluka analytical and Na 2 (SiO 2 ) 3,3 from Huber engineered materials (Zeopol 33). The amounts of reactants in the solution were defined to produce a layer with the average thickness of 8.4 nm. The density of the layer was expected to be 2.6 mg/m 3 according to the formula (CaO) 1,7 (SiO 2 )(H 2 O) 1.8 [24].
After precipitation of the C-S-H limestone fillers were immediately washed three times with 5 mM Ca(OH) 2 which was saturated in respect to silicon.
Washing was done in order to remove sodium. Both, coating and washing processes were monitored with conductometer (Radiometer analytical, CDM 2010) to ensure similar processing. Solution was separated by decantation and the excess water was evaporated from the slurry by drying at 40˚C until the water content was sufficiently low to produce pastes. The successfulness of coverage of limestone with C-S-H was verified by JEOL Scanning Electron Microscope  ( Figure 2). Pastes were prepared by mixing all dry materials for 1 min, followed by addition of the tempered water in the next 30 s. The total mixing time was 5 minutes. Samples were placed into a semi-adiabatic calorimeter 15 min after mixing. It was assumed that the initial heat of dissolution was lost and the observed heat originated from silicate based reactions [25]. Heat capacities were calculated and the experiments conducted according standard procedure [26] with two parallel samples. The experimental mixes are listed in Table 2.
The heat output rate at the increasing temperature of the semi-adiabatic conditions was normalized to constant temperature with Arrhenius equation using the value of 33 kJ/mol for the apparent activation energy [26] [27].
The end of the accelerating period was determined by maximum value of the heat rate, which denotes the end of accelerating period and beginning of decelerating period. This point was named as the inflection point.

Results
Addition of limestone fillers accelerated heat evolution in all studied samples.
Even greater acceleration was observed when C-S-H coated limestone was used.
The application of the autocatalytic model to the heat output revealed that the quantity of the initial C-S-H, [B 0 ] increased as the water/cement-ratio (w/c) increased ( Figure 4). Addition of limestone filler increased the total surface area to the mix but did not affect the quantity of the formed C-S-H. On the contrary  The heat output at the inflection point (point where the reaction rate began to decrease) increased as the w/c-ratio increased. Addition of limestone and C-S-H coated limestone appeared to decrease the heat output ( Figure 8).

Discussion
Although the experiments were made with white Ordinary Portland Cement and grounded limestone, the cement hydration followed the autocatalytic reaction     According to the autocatalytic modelling, the quantity of the initial C-S-H increased as the w/c-ratio increased in pure cement pastes. This observation complied with earlier studies, stating that the quantity of the initial C-S-H depends on the amount of the supersaturated silicon in the first dissolution step [6].
As limestone fillers were introduced to the mixes, the quantity of the C-S-H formed initially did not increase according to the autocatalytic modelling. However, in the presence of the C-S-H coated limestone filler, the increase was up to 4.5 times higher in comparison with the uncoated filler and pure paste. The result indicated that the C-S-H coating acted as nucleation sites from the very beginning of the measuring period.
Towards the end of the acceleratory period in the presence of the filler surfaces, the reaction propagated faster than could be expected according the autocatalytic reaction alone. The increased nonlinear propagation of the reaction was impossible to explain by increasing the quantity of C-S-H formed initially. Consequently, a deviation from the autocatalytic model in the reaction rate coefficient (k 2 ) was introduced. The higher reaction rate could be a consequence of an additional C-S-H nucleation which followed the primary nucleation. The additional C-S-H nucleation could take place simultaneously with the autocatalytic reaction. The difference is that C-S-H growth on C-S-H does not need additional energy but the C-S-H growth on mineral surfaces must overcome energy barrier.
This energy barrier is known in nucleation and crystal growth theories when materials precipitate on foreign substrate. Crystallization from solutions underlies in a large range of industrial, laboratory and physiological processes [29].The precipitation process over a certain energy barrier is expressed by Equation (4) [18]: I h is the rate of nucleation in a unit area of substrate. K h is a constant irrespective of the substrate type, presenting nucleation rate without energy barrier. R is the universal gas constant. T is a temperature and where ∆G k is the potential energy barrier for a homogeneous nucleation and θ is the contact angle between the nucleating crystal and the substrate. When the substrate is identical to nucleating crystal, θ = 0˚ and no potential barrier exist. If the substrate does not have any similarities to nucleating crystals, then θ = 180˚ and additional nucleation rate is similar to a homogeneous nucleation. According to the classic nucleation theory, the potential barrier for a homogeneous nucleation can be related to the degree of supersaturation as expressed in Equation (6), [31]: r is the radius of nucleating product, σ is the surface tension related to the product, υ is the molar volume of the product, Q is the ion activity product in the supersaturated solution and K sp is the solubility product of a calcium-silicate-hydrate.
The values of surface tension (σ) of a nucleating C-S-H, molar volume (υ) and radius (r) were determined by Gauffinet and Nonat [5]. The contact angle (θ) between calcite and C-S-H was calculated to be 60˚ [5], which further yields the relative nucleation rate (I h ) a value 0.13 at the equilibrium saturation. According to calculations, additional nuclides on filler surfaces are able to precipitate during the crystal growth of calcium silicate hydrates. The larger surface area of fillers in the mixes, the larger is the deviation of reaction rate k 2 (Figure 7). According to calculations, this deviation originates further nucleation of C-S-H on mineral filler surfaces.
Since the filler surfaces perform as active nucleation sites for the C-S-H formation the hydration products will be also deposited on these surfaces along with the OPC surfaces. Consequently, a formation of a thinner product layer on OPC could be expected in the presence of fillers. The total product volume should remain the same. The calorimetric test results revealed a decreasing heat evolution at the inflection point in the presence of fillers which contradicts the earlier assumption. The formed layer of hydration product on the OPC surface at the knee point (the time of the change of the reaction mode) appeared to consist of a smaller quantity of product per unit of OPC surface in the presence of fillers when compared to the pure paste. This observation contradicts the wide spread thinking that the knee point corresponds to the formation of a diffusion barrier. The present test results indicate that the origin of the knee point is different. One explanation could be related to the geometry of the particle system particularly the inter particle distances. Generally, the shorter is the distance the smaller is the heat output at the inflection point ( Figure 9) and the more limited is the space for C-S-H formation around OPC particles presuming that the particles are distributed evenly. The lack of a free volume around cement particles is related to the onset of the densification of C-S-H beyond a critical point that reduces its efficiency as an autocatalyst. Similar behavior was also observed when the w/c-ratio of pure pastes was varied (Figure 8). Densification of early C-S-H versus time has been observed with SANS measurements by Jennings et al. [32].
It has also been modelled by Bishnoi and Scrivener [33].

Conclusions
Early-age hydration was studied with semi-adiabatic calorimeter. Effects of temperature rise and heat capacities were normalized in a standard manner. For normalized calorimeter curves, autocatalytic model was fitted with two variable parameters, reaction rate and quantity of initial nuclei. It was observed that  . Inflection point and average particle distance.
cement hydration followed autocatalytic reaction principles throughout induction period and accelerating period. Autocatalytic model and measured calorimeter curve started to significantly deviate at the inflection point, where the reaction mode changed. According autocatalytic modelling this type of reaction mode change can be understood as a densification process. In most of the samples, beginning of densification process was observed to depend on the average particle distance. A simple method to apply autocatalytic reaction was established. It was assumed that autocatalytic material, C-S-H, was formed through supersaturation/precipitation-process during the initial dissolution. It was found to be an accurate approach in terms of modelling of pure cement pastes. As limestone fillers were introduced to the mixes, the quantity of initially formed C-S-H did not increase. In case of C-S-H coated fillers, a huge increase of initially formed C-S-H was observed. The reaction rate of the autocatalytic reaction was increased as additional surface was introduced to samples. The increased reaction rate was related to additional nucleation of C-S-H on the filler surface and simple thermodynamic equations were proposed to evaluate these effects. This type of approach could give a valuable data for cement hydration simulations and microstructural modelling [34] [35], especially in the case of evaluation of effect of supplementary cementitious materials on the early-age hydration.